In membrane separation, spent metal removal fluids are pumped from a process tank at a moderate pressure (typically 30 to 50 PSIG) and rapid flow
to a series of membranes. This flow is typically between 750 to 1,100 gallons per square foot of membrane per day and is referred to as the feed rate.
Large molecules and virtually all petroleum products are blocked at the membrane surface. The compounds that do not pass through the membrane are
referred to as the reject. The water-like solutions that pass through the membrane are referred to as the "permeate". The rate at which the permeate
flows through the membrane is called the flux rate. More information about the flux rate was given below. You can find info about that how it is measured
and when it becomes unacceptably low.
Flux Rate...
The permeate rate is measured as a direct function of the available square footage of membrane. The permeate rate is referred to as the "flux rate"
and is measured as gallons per square foot of available membrane area per day. This is typically abbreviated as GSFD. Flux rates are between 20
and 40 GSFD. The more concentrated the feed solution, the slower the flux rate. The remaining rejected spent metal removal solution, the difference
between the feed rate and the permeate rate, is returned to the process tank for recirculation, retreatment, and concentration. The success of operating
a very tight membrane, 0.005 micron pore size, in a relatively dirty environment (spent metal removal fluids) is due to a very high feed rate as a function
of permeate rate. This ratio is in the range of 30 gallons feed to one gallon permeate. The remaining 29 gallons are used as a high flow across the membrane
surface. This high cross flow rate essentially allows the membrane to be self-cleaning. The recirculated fluids continue to concentrate until the flux rate drops
to an unacceptable level. This unacceptable flux rate is typically between 5 and 8 GSFD. At this point, the concentrated recirculated fluid is transferred to a
separate tank for contract disposal. The spent metal removal solutions can be concentrated from 30% to 50% (volume/volume) oil.
Membrane Types...
Membranes come in two basic sizes : ( a ) Microfiltration, rated at 0.1 to 1.0 micron and ( b ) Ultrafiltration, rated from 0.001 to 0.1 micron, the most
typical membrane size rate at 0.005 micron. Membranes can be configured in various ways and have varying life spans : ( a ) Round tubes with
approximately 0.5" or 1" internal diameter, which can last from 3 to 8 years, ( b ) Hollow fibers with an approximate internal diameter of 0.030", which
can last from 1 to 2 years, ( c ) Flat sheets wrapped in a spiral configuration, lasting from 3 to 8 years and ( d ) Flat sheets that are vibrated or turbulated
with mechanical "wipers," lasting from 3 to 8 years.
Operating cost, including membrane depreciation over time, is between 0.5 and 1.0 cents per gallon. Some companies are now using ultrafilter membranes
followed by nanoporous filter (NF) membranes or reverse osmosis ( RO ) membranes. These membranes are more sensitive to fouling than ultrafilter
membranes, even though the NF membranes or the RO membranes only see the permeate from the ultrafilter. The treated water after two stage UF/NF or
UF/RO can be suitable for re-use in other industrial process water requirements such as non-critical parts washing and rinsing.
An influent storage tank, a processing tank, and an oil-hauling tank are necessary. The processing tank should be equal to one-half of the daily average flow.
Truck spill containment may be required for the oil hauling tank pump-outs by a contract service. You can find more info about details of the characteristics of
pollutants after ultrafiltration separation below.
Pollutant
BOD5 ( mg / L )
COD ( mg / L )
O & G ( mg / L )
pH
1
200
600
55
8.7
2
230
700
70
9.0
3
290
1,200
120
8.8
4
250
5,500
95
9.3
5
250
5,800
90
9.2
6
430
25,000
80
9.2
Advantages and Disadvantages of the Membrane Separation...
Advantages : ( a ) Membrane separation consistently separates a wide variety of emulsion, surfactant, and chelating chemistries and various mixtures.
( b ) It requires no specific chemical knowledge. ( c ) Complex instrumentation is not required. ( d ) The method does not require constant attention. ( e )
The basic concept is simple to understand.
Disadvantages : ( a ) Membranes are expensive. ( b ) Certain solvents can quickly and permanently destroy the membrane. ( c ) Certain colloidal
solids, especially graphite and residues from vibratory deburring operations, can permanently foul the membrane surface. ( d ) The energy cost is higher
than chemical treatment, although less than evaporation. ( e ) Oil emulsions are not "chemically separated," so secondary oil recovery can be difficult. ( f )
Synthetics are not effectively treated by this method.
